Motor starter apparatus with start-up fault detection capability

ABSTRACT

A motor starter apparatus includes at least one semiconductor switch configured to selectively couple a power source to a motor, at least one current sensor configured to generate a current sense signal indicative of a current provided via the at least one semiconductor switch, and a control circuit coupled to the at least one current sensor and configured to cause the at least one semiconductor switch to momentarily couple the power source to the motor and identify a fault based on a behavior of the current sense signal in response to the momentary coupling. The control circuit may be configured to identify the fault responsive to detecting that a rate of change of the current in response to the momentary coupling meets a predetermined criterion.

BACKGROUND

The inventive subject matter relates to motor control apparatus andmethods of operation thereof, more particularly, to motor starters andmethods of operating the same.

Motor starters are commonly used with industrial electric motors.Typical solid-state motor starters control starting characteristics tomeet application requirements, including acceleration and decelerationtime, starting current and motor torque. Soft starters are commonly usedto limit inrush current when the motor is first coupled to a powersource. Large inrush currents may cause voltage dips that may negativelyaffect other loads coupled to the same source. High starting torque mayalso cause electromechanical shock that can damage windings and othercomponents of the motor, as well as drive trains and other componentsmechanically coupled to the motor.

Soft motor starters may control voltage and/or current provided to themotor. For example, some soft starters may use silicon-controlledrectifiers (SCRs) that are connected in series between the power sourceand the load. During a startup process, the SCRs may be selectivelygated “on” to gradually increase the speed of the motor. When the motorhas reached a desired speed, a bypass contactor may be closed to bypassthe SCRs and reduce losses. Operations of soft motor starters aredescribed in “Solid-state soft start motor controller and starter,”Application Paper AP 03902001E (Eaton Corp. 2011).

SUMMARY

Some embodiments of the inventive subject matter provide a motor starterapparatus including at least one semiconductor switch configured toselectively couple a power source to a motor, at least one currentsensor configured to generate a current sense signal indicative of acurrent provided via the at least one semiconductor switch, and acontrol circuit coupled to the at least one current sensor andconfigured to cause the at least one semiconductor switch to momentarilycouple the power source to the motor and to identify a fault based on abehavior of the current sense signal in response to the momentarycoupling. The control circuit may be configured to identify the faultresponsive to detecting that a rate of change of the current in responseto the momentary coupling meets a predetermined criterion.

In some embodiments, the control circuit may be configured to detectthat the rate of change of the current in response to the momentarycoupling meets the predetermined criterion responsive to a magnitude ofthe current sense signal exceeding a predetermined magnitude. Thecontrol circuit may be configured to detect that the rate of change ofthe current in response to the momentary coupling meets thepredetermined criterion responsive to a comparison of the current sensesignal to a reference signal, which may be user adjustable.

Further embodiments provide a motor starter apparatus including at leastone semiconductor switch configured to selectively couple a power sourceto a motor, at least one current sensor configured to sense a currentprovided from the power source, and a control circuit coupled to the atleast one current sensor and configured to cause the at least onesemiconductor switch to momentarily couple the power source to the motorand to identify a fault based on a comparison of a reference signal to acurrent sense signal generated by the current sensor in response to themomentary coupling. The control circuit may include a comparison circuitconfigured to generate a fault indication signal responsive to thecomparison of the reference signal to the current reference signal. Thecomparison circuit may include a window comparator circuit having adetection window defined by first and second reference signals. Thecontrol circuit may further include a reference generator circuitconfigured to generate the first and second reference signals.

Further embodiments provide methods of operating a motor starter. Themethods include the method operating at least one semiconductor switchof the motor starter to momentarily couple a power source to a motor,responsively generating a current sense signal indicative of a currentprovided via the at least one semiconductor switch, and identifying afault based on a behavior of the current sense signal generated inresponse to the momentary coupling.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a motor starter apparatusaccording to some embodiments of the inventive subject matter.

FIG. 2 is a schematic diagram illustrating a motor starter apparatusaccording to further embodiments.

FIG. 3 is a flowchart illustrating operations of the apparatus of FIG. 2according to some embodiments.

FIG. 4 is a schematic diagram illustrating a current detection circuitaccording to some embodiments.

FIG. 5 is a schematic diagram illustrating a current reference signalgenerator circuit according to some embodiments.

FIG. 6 is a flowchart illustrating operations of the apparatus of FIG. 2according to further embodiments.

DETAILED DESCRIPTION

Specific exemplary embodiments of the inventive subject matter now willbe described with reference to the accompanying drawings. This inventivesubject matter may, however, be embodied in many different forms andshould not be construed as limited to the embodiments set forth herein;rather, these embodiments are provided so that this disclosure will bethorough and complete, and will fully convey the scope of the inventivesubject matter to those skilled in the art. In the drawings, likenumbers refer to like items. It will be understood that when an item isreferred to as being “connected” or “coupled” to another item, it can bedirectly connected or coupled to the other item or intervening items maybe present. As used herein the term “and/or” includes any and allcombinations of one or more of the associated listed items.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the inventivesubject matter. As used herein, the singular forms “a”, “an” and “the”are intended to include the plural forms as well, unless expresslystated otherwise. It will be further understood that the terms“includes,” “comprises,” “including” and/or “comprising,” when used inthis specification, specify the presence of stated features, integers,steps, operations, items, and/or components, but do not preclude thepresence or addition of one or more other features, integers, steps,operations, items, components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this inventive subject matterbelongs. It will be further understood that terms, such as those definedin commonly used dictionaries, should be interpreted as having a meaningthat is consistent with their meaning in the context of thespecification and the relevant art and will not be interpreted in anidealized or overly formal sense unless expressly so defined herein.

Some embodiments of the inventive subject matter arise from arealization that industrial electric motors may employ long cablesand/or power factor correction capacitors that can have highcapacitances associated therewith, and such high capacitance may causefailure of solid-state motor starters during start-up due to anexcessive rate of change of the current (di/dt). Stray capacitancebetween conductive in a motor cable may not be known, and power factorcorrection capacitors may be inadvertently left connected at startup,which can result in failure of switching devices (e.g., SCRs) of a motorstarter when the starter is activated. Some embodiments of the inventivesubject matter may use a current rate of change detector that may beused to detect such start-up fault conditions and prevent starteroperation that may result in damage. In some embodiments, an SCR of amotor starter may be fired at a relatively small angle to produce anarrow voltage pulse. A current produced by this pulse may be detectedusing a high-speed current sensor, such as a ferrite current transformeror Rogowski coil sensor. The current sense signal produced by the sensormay be compared to a reference signal. The current sense signalexceeding this reference signal may indicate an excessive current rateof change that may be associated with the presence of a power factorcorrection capacitor, excessive line capacitance or other faultcondition. A fault indication signal may be asserted based on thecomparison and used to provide an alert to an operator and/or inhibitfurther operation of the starter to prevent damage.

FIG. 1 illustrates a motor starter 100 according to some embodiments ofthe inventive subject matter. The starter 100 includes at least onesemiconductor switch 110 and at least one bypass switch 120, each ofwhich is configured to couple a power source 10 to a motor 20. A controlcircuit 130 controls the at least one semiconductor switch 110 and theat least one bypass switch 120. The control circuit 130 may beconfigured to provide a soft start operation in which the at least onesemiconductor switch 110 is modulated while the bypass switch is open togradually accelerate the motor 20. Once the motor 20 has reached adesired state, the at least one bypass switch 120 may be closed and theat least one semiconductor switch 110 opened.

As further illustrated, the control circuit 130 may be configured toimplement a startup test controller 132 that operates the at least onesemiconductor switch 110 in a constrained before proceeding with astartup procedure to detect current levels that indicate conditions thatmay cause damage to the starter 100 during startup. As explained above,output line and/or load conditions for the starter 100 may cause currentrate of change levels associated with fault conditions that may damagethe starter 100 (e.g., the at least one semiconductor switch 110) duringstartup. For example, such fault conditions may include inadvertentconnection of switchable power factor correction capacitors in parallelwith the motor 20, excessive capacitance between conductors ofrelatively long cables (e.g., in undersea applications) and/or shortcircuits.

The startup test controller 132 may modulate the at least onesemiconductor switch 110 to generate a controlled-duration voltage pulsethat generates a current i that can be monitored to determine whethersuch a damaging load condition is present, without causing currentlevels that may damage the starter 100. For example, if a rate of changeof the current i meets a predetermined criterion (e.g., the current iexceeds a predetermined threshold in response to the limited-durationvoltage pulse), such a condition may indicate the presence of a fault,such as a short, excessive load capacitance, excessive line capacitance,or the like. If a fault condition is detected, the fault condition maybe indicated using, for example, a fault indication signal that may betransmitted to an operator. Such a fault indication signal may also beused to inhibit operation of the starter 100 and prevent damage to thestarter 100.

FIG. 2 illustrates an example of a starter 200 according to furtherembodiments. The starter 200 includes at least one semiconductor switch212 including anti-parallel connected silicon-controlled rectifiers(SCRs). Gate terminals of the SCRs of the at least one semiconductorswitch 212 may be driven by a gate drive circuit 232 of a controlcircuit 230. The gate drive circuit 232 is controlled by a controller231, which may include, for example, a DSP-based microcontroller orsimilar device. The controller 231 may also control a contactor drivecircuit 234 that drives a bypass contactor 220.

The starter 200 further includes at least one current sensor 240, whichsenses a current passing between a power source 10 and a motor 20. Theat least one current sensor 240 may include at least one high-frequencycurrent sensor, such as a ferrite current transformer or a Rogowski coilcurrent sensor. The control circuit 230 further includes a detectorcircuit 233, which is configured to receive at least one current sensesignal generated by the at least one current sensor 240 and toresponsively generate a fault indication signal that is provided to thecontroller 232. As illustrated, the detector circuit 233 may generatethe fault indication signal responsive to a comparison of the currentsense signal to a reference signal provided by the controller 231.

FIG. 3 illustrates exemplary operations of the starter 200 according tosome embodiments. Responsive to a start command (block 310), thecontroller 231 may gate the SCRs of the at least one semiconductorswitch 212 at a relatively low phase angle to apply a momentary voltagepulse at the output of the starter 200 (block 320). The detector circuit233 may compare the current reference signal to a current sense signalgenerated by the current sensor 240 in response to the voltage pulse(Block 330). If the current sense signal exceeds the limit defined bythe reference signal, indicating an undesirable current rate of changecorresponding to a short circuit and/or excessive capacitance, thedetector circuit 233 may generate a fault indication signal (block 360).The fault indication signal may be used to inhibit motor starting (block370). For example, the fault indication signal may be provided to thecontroller 231, which may responsively inhibit activation of the gatedrive circuit 232. If the sensed current is not excessive, the startupprocedure may proceed (block 340). After startup is accomplished, thebypass contactor 220 may be closed and the at least one semiconductorswitch 210 opened (block 350).

FIGS. 4 and 5 illustrate a window comparator circuit 400 and referencegenerator circuit 500 that may be used in the detector 233 of FIG. 2.The window comparator circuit 400 includes first and second comparators410 a, 410 b that are configured to receive a positive current thresholdsignal I_TH+ and a negative current threshold signal respectively, whichdefine a detection window. The comparators 410 a, 410 b compare acurrent sense signal I_IN to the respective positive and negativecurrent threshold signals I_TH+, I_TH−. If the current sense signal isoutside of the detection window, i.e., greater than the positive currentthreshold signal I_TH+ or less than the negative current thresholdsignal I_TH−, a fault signal I_FAULT is driven to a logic “low”,indicating a current condition associated with a possible fault. If thecurrent sense signal I_N stays within the limits defined by the positiveand negative current threshold signals I_TH+, I_TH−, the fault signalI_FAULT maintains a logic “low” state, indicating absence of a faultcondition. As described above, a fault condition may include, forexample, the presence of power factor correction capacitors, excessiveline capacitance and/or a short circuit. The controller 231 may monitorthe fault indication signal I_FAULT and responsive take further actionsbased on the state of the fault indication signal I_FAULT. For example,if the controller 231 is microcontroller-based, the fault indicationsignal I_FAULT may be used as an interrupt, with assertion of theinterrupt triggering generation of an alarm signal and/or inhibition offurther operations, as described above.

Referring to FIG. 5, the reference generator circuit 500 generates thepositive and negative current threshold signals I_TH+, I_TH− responsiveto a reference signal REF. In particular, the reference generatorcircuit 500 includes first and second operational amplifiers 510 a, 510that generate respective ones of the positive and negative currentthreshold signals I_TH+, I_TH− from the reference signal REF. Thereference signal REF may be provided by the controller 213, and may beuser adjustable responsive to a control input, such as a user-generatedcommand.

In three-phase implementations according to some embodiments, differenttypes of such pulse tests may be conducted to detect line-to-line orline-to-ground conditions. Referring to FIG. 6, in response to a startcommand (block 610), individual phases may be pulsed to testline-to-ground conditions (block 620). If any of the current pulsesexceeds a first predetermined threshold, a fault indication signal maybe asserted (blocks 630, 680). If not, pairs of phases may be pulse totest line-to-line conditions (block 640). If the currents generated byany of these pulses exceed a second predetermined threshold, a faultindication signal may be asserted (blocks 650, 680). The first andsecond thresholds may be the same or different. If both tests arepassed, a start up procedure may be performed (block 660), concludingwith closure of the bypass contactor 220. If either test is failed,startup may be inhibited (block 690).

In the drawings and specification, there have been disclosed exemplaryembodiments of the inventive subject matter. Although specific terms areemployed, they are used in a generic and descriptive sense only and notfor purposes of limitation, the scope of the inventive subject matterbeing defined by the following claims.

That which is claimed:
 1. A motor starter apparatus comprising: at leastone semiconductor switch configured to selectively couple a power sourceto a motor; at least one current sensor configured to generate a currentsense signal indicative of a current provided via the at least onesemiconductor switch; and a control circuit coupled to the at least onecurrent sensor and configured to cause the at least one semiconductorswitch to momentarily couple the power source to the motor and toidentify a fault responsive to detecting from the current sense signalthat a rate of change of the current in response to the momentarycoupling meets a predetermined criterion.
 2. The apparatus of claim 1,wherein the control circuit is configured to detect that the rate ofchange of the current in response to the momentary coupling meets thepredetermined criterion responsive to a magnitude of the current sensesignal exceeding a predetermined magnitude.
 3. The apparatus of claim 1,wherein the control circuit is configured to detect that the rate ofchange of the current in response to the momentary coupling meets thepredetermined criterion responsive to a comparison of the current sensesignal to a reference signal.
 4. The apparatus of claim 3, wherein thereference signal is user adjustable.
 5. The apparatus of claim 1,wherein in the control circuit is further configured to inhibitoperation of the at least one switch responsive to identification of thefault.
 6. The apparatus of claim 1, wherein the at least onesemiconductor switch comprises a silicon controlled rectifier (SCR) andwherein the control circuit is configured to fire the SCR at apredetermined angle to generate a voltage pulse of a predeterminedduration during the momentary coupling.
 7. A motor starter apparatuscomprising: at least one semiconductor switch configured to selectivelycouple a power source to a motor; at least one current sensor configuredto sense a current provided from the power source; and a control circuitcoupled to the at least one current sensor and configured to cause theat least one semiconductor switch to momentarily couple the power sourceto the motor, to identify a fault based on a comparison of a referencesignal to a current sense signal generated by the current sensor inresponse to the momentary coupling, and to inhibit operation of the atleast one semiconductor switch responsive to identification of thefault.
 8. The apparatus of claim 7, wherein the control circuitcomprises a comparison circuit configured to generate a fault indicationsignal responsive to the comparison of the reference signal to thecurrent reference signal.
 9. The apparatus of claim 8, wherein thecomparison circuit comprises a window comparator circuit having adetection window defined by first and second reference signals.
 10. Theapparatus of claim 9, wherein the control circuit further comprises areference generator circuit configured to generate the first and secondreference signals.
 11. The apparatus of claim 7, wherein the referencesignal is user adjustable.
 12. The apparatus of claim 7, wherein the atleast one semiconductor switch comprises a silicon controlled rectifier(SCR) and wherein the control circuit is configured to fire the SCR at apredetermined angle to generate a voltage pulse of a predeterminedduration.
 13. A method of operating a motor starter, the methodcomprising: operating at least one semiconductor switch of the motorstarter to momentarily couple a power source to a motor; responsivelygenerating a current sense signal indicative of a current provided viathe at least one semiconductor switch; and identifying a faultresponsive to detecting that a rate of change of the current in responseto the momentary coupling meets a predetermined criterion.
 14. Themethod of claim 13, wherein identifying the fault responsive todetecting that a rate of change of the current in response to themomentary coupling meets a predetermined criterion comprises detectingthat a magnitude of the current sense signal exceeds a predeterminedmagnitude.
 15. The method of claim 13, wherein identifying the faultresponsive to detecting that a rate of change of the current in responseto the momentary coupling meets a predetermined criterion comprisescomparing the current sense signal to a reference signal.
 16. A motorstarter apparatus comprising: at least one semiconductor switchconfigured to selectively couple a power source to a motor; at least onecurrent sensor configured to sense a current provided via the at leastone semiconductor switch; and a control circuit configured to cause theat least one semiconductor switch to apply a voltage pulse to the motorand to inhibit startup of the motor following application of the voltagepulse if a magnitude of the sensed current exceeds a predetermined limitin response to the voltage pulse.
 17. The motor starter apparatus ofclaim 16, wherein the predetermined limit defines a limit of a rate ofchange of the current.
 18. The motor starter apparatus of claim 16,wherein the at least one switch comprises an SCR and wherein the controlcircuit is configured to fire the SCR once to generate the voltage pulseand to inhibit further firing of the SCR if the magnitude of the sensedcurrent exceeds the predetermined limit in response to the voltagepulse.